Optical transmission system and optical transmission method

ABSTRACT

An OADM device without a transponder unit does not mount the transponder in charge of separating a fault between the OADM device and an external device and is difficult to perform fault separation and to identify a fault interval. To solve this problem, for the OADM device without the transponder unit, the optical loop back function is provided by using such a switch as, for example, 2×2 optical switch. By identifying a fault developing interval by the use of the optical loop back function, fault separation at the time of occurrence of a fault can be facilitated.

INCORPORATION BY REFERENCE

The present application claims priority from Japanese applicationJP2010-085636 filed on Apr. 2, 2010, the content of which is herebyincorporated by reference into this application

BACKGROUND OF THE INVENTION

The present invention relates to a transmission apparatus and atransmission method for transmitting optical signals subject tomultiple-wavelength.

In order to reduce system costs while increasing the capacity ofcommunications, a multiple-wavelength optical transmission technique forcommunicating a plurality of signal light rays of different wavelengthsby tying them up into a single optical fiber has generally been appliedto an optical communication system. In an actual system, amultiple-wavelength functional device, that is, a wavelength multiplexerdevice is installed which is constituted by a light ray insert unitadapted to bundle a plurality of signal light rays of differentwavelengths into a single optical fiber, a light ray split unit adaptedto split, wavelength by wavelength, the bundled plural signal lightrays, and an optical fiber amplifier unit adapted to compensate anoptical signal for a loss generated in an optical fiber representing atransmission path between two remotely distant locations. Provided forthe wavelength multiplexer device is a transponder which converts alight signal into that having a wavelength suitable formultiple-wavelength. By using the wavelength multiplexing device and aninterface device in combination, inexpensive communication can beoffered over a long distance.

Reference is now made to FIG. 1 illustrating a general networkconfiguration using a multiple-wavelength transmission system.Structurally, the network includes an access quarter 1-4 for offeringFTTH (Fiber To The Home) service to subscribers in a unit of areas byusing an OLT (Optical Line Terminal) unit 1-2 and an ONU (OpticalNetwork Unit) 1-3, an edge quarter 1-6 for concentrating communicationsfrom subscribers in area units into a group of areas by using a switch1-5 of a plurality of L2's (Layers 2), a metro quarter 1-7 forconcentrating the communications concentrated by the L2 (Layer 2) switchinto a unit of towns and cities, and a core quarter 1-8 for efficientlytransmitting a large capacity of communications concentrated in a unitof towns and cities over a long distance between metropolitans. In thepresent network, an ODAM (Optical Add Drop Multiplexer) device asdesignated at reference numeral 1-1 represents an optical transmissionsystem used for concentrating into a single location the communicationsscattered in a relatively wide range.

Turning to FIG. 2, the OADM device 1-1 is generally constructed asexemplified therein. In FIG. 2, two OADM devices 1-1 are so configuredas to oppose to each other with intervention of two transmission paths2-1. The OADM device 1-1 includes a wavelength multiplexer unit 2-5adapted either to take out a desired signal from a plurality of lightsignals subject to multiple-wavelength or to pile the desired signal upagain into a group of plural signals by causing the desired signal toundergo multiple-wavelength, a transponder unit 2-3 adapted to eithersuitably convert a signal split from the wavelength multiplexer unit 2-5by discriminating it from a subscriber signal accommodated in the OADMdevice 1-1 or to suitably convert the subscriber signal and cause it toundergo multiple-wavelength by means of a splitter/inserter, and asupervisory control unit 2-4 adapted to perform supervisory control ofthe wavelength multiplexer unit 2-5 and transponder unit 2-3.

A supervisory control light controlling unit 2-2 causes a signal lightray 2-7 outputted from the transponder unit 2-3 and a supervisorycontrol tight ray 2-6 outputted from a supervisory control lightprocessing unit 2-4-1 under the control of the supervisory control unit2-4 to undergo multiple-wavelength and transmits them to thetransmission path 2-1. Also, the supervisory control light controllingunit 2-2 causes the signal light ray 2-7 subject to multiple-wavelengthand received from the transmission path 2-1 and the supervisory lightray 2-6 from the supervisory control light processing unit 2-4-1 toundergo wavelength splitting, respectively, and the individual splitlight rays are inputted to the transponder unit 2-3 and the supervisorycontrol unit 2-4, respectively.

In the OADM device 1-1 shown in FIG. 2, the signal light ray 2-7propagates through a path indicated at dotted line and the supervisorycontrol light ray 2-6 propagates from the supervisory control lightprocessing unit 2-4-1 through a path as indicated at solid line. Moreparticularly, the light ray from the supervisory control lightprocessing unit 2-4-1 is split by means of the supervisory control lightcontrolling unit 2-2 in the opposing OADM device 1-1 and inputted to thesupervisory control light processing unit 2-4-1 in the opposing OADMdevice 1-1. The signal light ray 2-7 passes through the supervisorycontrol light controlling unit 2-2 and is then inputted to thewavelength multiplexer unit 2-5.

Turning to FIG. 3, an explanation will diagrammatically be given to amultiple-wavelength light ray having a signal wavelength group 3-2subject to multiple-wavelength and a supervisory control signal 3-1. Ina general OADM device 1-1, there exist, as described inJP-A-2003-046456, for example, the signal group of plural signalssubject to multiple-wavelength carrying actual communication data andthe supervisory control signal 3-1 adapted to materialize communicationof signals used for executing mutual control and supervisory of the OADMdevices 1-1 located remotely. In FIG. 3, the signal wavelength group 3-2of 32 signals subject to multiple-wavelength and the single supervisorycontrol signal 3-1 arranged on the side of short waves are illustratedbut when the supervisory control signal 3-1 arranged on the side of longwaves or when a plurality of supervisory control signals 3-1 arearranged, a number other than 32 of signal waves will sometimes besubjected to multiple-wavelength.

In the general OADM device 1-1, the function to take out a desiredsignal from a plurality of optical signals or executemultiple-wavelength of the desired signal and to pile it up into thegroup of plural signals is executed in the wavelength multiplexer unit2-5. For communication of the supervisory control signal between theremotely located OADM devices 1-1, only the supervisory control signalis split from the light rays subject to multiple-wavelength by means ofthe supervisory control light controlling unit 2-2 arranged in the inputport from the transmission path 2-1, for example, and is then insertedor merged to the signal wavelength by means of the supervisory controllight controlling unit 2-2 arranged in the output to port of thetransmission path 2-1.

Reference is now made to FIG. 4 to indicate how an external device 4-1such as a router is connected to the transponder unit 2-3 and thewavelength multiplexer unit 2-5. The transponder unit 2-3 is comprisedof a multiple-wavelength side interface section 2-3-1 and an externaldevice side interface section 2-3-4. The multiple-wavelength sideinterface section has a transmitter 2-3-2 and a receiver 2-3-3. Thetransmitter 2-3-2 converts an optical signal to that having a wavelengthsuitable for multiple-wavelength and sends it to the wavelengthmultiplexer unit 2-5 and the receiver 2-3-3 receives an optical signalsent from the wavelength multiplexer unit 2-5. Similarly, the externaldevice side interface section 2-3-4 has a transmitter 2-3-6 and areceiver 2-3-5 to thereby receive an optical signal from the externaldevice 4-1 or transmit an optical signal thereto. Similarly, theexternal device 4-1 has a transmitter 4-2 for transmission to thetransponder unit 2-3 and a receiver 4-3 for reception from thetransponder unit 2-3. Since the signal transmitted from the externaldevice 4-1 does not always have a wavelength suitable formultiple-wavelength. an optical signal transmitted from the transmitter4-2 of external device 4-1 cannot be inputted to the wavelengthmultiplexer unit 2-5 while keeping its wavelength intact.

It is necessary for the OADM device 1-1 of the general construction asabove to add a transponder unit 2-3 each time it connects to theexternal device 4-1. As a result, in case a plurality of externaldevices 4-1 are connected, the number of transponder units 2-3 to beinstalled must be increased and consequently, the cost of the systemrises.

To cope with this problem, the wavelength of a signal outputted from thetransmitter 4-2 of external device 4-1 is made in advance suitable formultiple-wavelength to permit connection to the wavelength multiplexerunit 2-5 without resort to the transponder unit 2-3, thus reducing thesystem cost.

The transponder unit 2-3, however, does not function merely to convertthe wavelength of optical signal into that suitable formultiple-wavelength but is used to separate and identify a locationwhere a fault occurs inside the network using the OADM device 1-1 andtherefore, the aforementioned expedient needs a construction forseparating a fault and identifying a fault interval during occurrence ofthe fault, eventually raising a problem that the running cost increasesand as a result, the total cost is raised.

In order to facilitate handling the fault generation in the OADM devicewithout the transponder unit 2-3 as described above, two countermeasuresof 1) intensifying the function of supervisory the external device 4-1has and 2) intensifying the function of supervisory the OADM device 1-1has are conceivable.

Of them, in connection with the method for intensifying the supervisoryfunction provided for the external device 4-1 as in 1), a method ofintensifying the function to supervise a physical layer such as foroptical input/output supervisory provided for the external device 4-1and a method of intensifying the function to supervise a data link layerand a network layer such as Inthernet (registered trade mark) OAM, MPLSOAM are conceivable. Any of the supervisory functions, however, does notsupervise the inside of OADM device but gives the end-to-end supervisoryfunction executed externally of the OADM device. Therefore, when a faultoccurs inside the network using the OADM device, specified informationcannot be obtained as to where the fault occurs inside the OADM deviceand how the fault is to be dealt with. In other words, so long as themere end-to-end supervisory from the outside is executed, the faultdeveloping inside the OADM device cannot be separated specifically.

Further, solving the present problem by the provision of the newfunction for the external device 4-1 means that the external device 4-1having already been introduced into the network cannot be utilizedbecause it lacks the new function.

In connection with the method of intensifying the supervisory functionthe OADM device 1-1 has as mentioned in 2) above, the presence/absenceof a fault can be confirmed in respect of each optical signal byproviding the function to supervise individual optical signalsundergoing multiple-wavelength and accordingly, the supervisory functioncan be intensified. But the supervisory function as above needs to beprovided by the number of accommodated multiple-wavelengths, making theconstruction complicated and raising the cost of the OADM device per se.Further, by providing the function to analyze optical characteristics asin the case of an optical spectrum analyzer, supervisory of each opticalsignal can be intensified but the construction becomes complicated andbesides, because of very high expensiveness of the optical spectrumanalyzer, the cost of the OADM device 1-1 per se still arises.

As will be seen from the foregoing, in the OADM device without thetransponder unit 2-3, separation of a fault developing in the networkusing the OADM device 1-1 must be realized by avoiding complexity andexpensiveness.

SUMMARY OF THE INVENTION

In identifying a fault developing location in the OADM device devoid ofthe transponder unit, the optical loop back function is used by the useof an inexpensive optical switch of, for example, 2×2 (2 inputs, 2outputs to decide the presence/absence of occurrence of a fault.

Since the optical loop back can be carried out at a plurality oflocations in the OADM device 1-1, the presence/absence of optical loopback light rays can be confirmed in sequence while executingsequentially the optical loop back at the plural locations. Accordingly,by observing an optical signal subject to loop back, an interval inwhich a fault develops can be identified and an action to recover fromthe fault can be taken.

Exemplarily, an optical transmission apparatus according to the presentinvention comprises a wavelength multiplexer unit for multiplexingwavelengths of input/output light rays, a first optical switch forinputting/outputting light rays to/from the wavelength multiplexer unit,a control light processing unit for inputting/outputting control lightrays, a control light controlling unit for performing multiplexing orsplitting between the control light ray and the light ray delivered outof the wavelength multiplexer unit, and a supervisory control unit forcontrolling on/off of the function to fold the inputted light ray inrespect of the first optical switch. Then, the optical transmissionapparatus may further comprise a second optical switch arranged betweenthe control light processing unit and the control light controlling unitand the supervisory control unit may further control on/off of thefunction to fold the inputted light ray in respect of the second opticalswitch. Further, the optical transmission apparatus may be connected toanother optical transmission apparatus through a transmission paths tomutually transmit/receive light rays to/from the different opticaltransmission apparatus

Exemplarily, an optical transmission method according to the presentinvention is based on an OADM device having a wavelength multiplexerunit, a first optical switch, a control light processing unit and acontrol light controlling unit and comprises a step of multiplexingwavelengths of input/output light rays by means of the wavelengthmultiplexer unit, a step of switching an light ray delivered out of thewavelength multiplexer unit by means of the first optical switch, a stepof causing the control light controlling unit to multiplex the light rayswitched by the first optical switch with the control light ray thecontrol light processing unit outputs and transmitting a multiplexedlight ray to an opposing OADM device, a step of causing the controllight controlling unit to split a light ray received from the opposingOADM device into a control light ray and another light ray, and a stepof inputting the different light ray to the first optical switch tocause it to switch the different light ray, wherein when the firstoptical switch has its input light folding function turned on, theinputted light ray is folded and transmitted to an input originator.Then, the optical transmission method may further comprise a step ofcausing a second optical switch arranged between the control lightprocessing unit and the control light controlling unit to performswitching between the control light ray delivered out of the controllight processing unit and another light ray split by means of thecontrol light controlling unit, wherein when the second optical switchhas its input light folding function turned on, the inputted light rayis folded and transmitted to the input originator.

According to the teachings of the present invention as above, in theOADM device without the transponder unit 2-3, a fault developing insidethe network constituted by the OADM device can be separated easily whileavoiding expensiveness due to complexity of the construction.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an example of schematic configurationof the whole network.

FIG. 2 is a block diagram illustrating an example of schematicconstruction of a general OADM device.

FIG. 3 is a diagram showing an example of a multiple-wavelength signal.

FIG. 4 is a block diagram illustrating an example of connection of theOADM device to an external device.

FIG. 5 is a block diagram illustrating an example of an OADM devicedevoid of a transponder unit.

FIG. 6 is a block diagram illustrating an example of the OADM devicecombined with a supervisory control terminal for remote supervisorycontrol.

FIG. 7A is a diagram for explaining the operation of an optical switchadapted to materialize the optical loop back function.

FIG. 7B is a diagram for explaining the operation of an optical switchadapted to materialize the optical loop back function.

FIG. 8A is a block diagram illustrating an example of an OADM deuce withthe optical loop back function.

FIG. 8B is a block diagram illustrating an example of an OADM devicewith the optical loop back function.

FIG. 9 is a block diagram useful to explain a status in which a faultoccurs.

FIG. 10 is a block diagram for explaining the operation of the opticalswitch when a fault develops in the OADM device with the optical loopback function.

FIG. 11 is a block diagram for explaining the operation of the opticalswitch when a fault develops in the OADM device with the optical loopback function.

FIG. 12 is a block diagram illustrating an example of configuration setup when a light source for loop back is provided for the OADM devicewith the optical loop back function.

FIG. 13 is a block diagram showing flow of signals in the configurationset up when the light source for loop back is provided for the OADMdevice with the optical loop back function.

FIG. 14 is a block diagram illustrating a configuration formaterializing the optical loop back function in respect of a pluralityof OADM devices.

FIG. 15 is a block diagram for explaining the function of an opticalsplitter unit.

FIG. 16 is a block diagram for explaining the function of the opticalswitch unit.

DESCRIPTION OF THE EMBODIMENTS

Reference is now made to FIG. 5 useful to explain as a first embodimentan OADM device which uses 2×2 optical switches, lacks a transponder unitand has a wavelength multiplexer unit 2-5 directly connected with anexternal device 4-1. In the figure, a plurality of external devices 4-1are connected to a single OADM device but for simplicity ofillustration, reference numerals 4-1, 4-2 and 4-3 are designated to therightmost external device connected to the single OADM device. Thisholds true for illustration of the external device 4-1 in the followingdrawings. A signal light ray 2-7 outputted from the transmitter 4-2 ofexternal device 4-1 is inputted to the wavelength multiplexer unit 2-5and then it passes through a 2×2 optical switch 5-1. Further, asupervisory light ray 2-6 outputted from the supervisory control lightprocessing unit 2-4-1 passes through a 2×2 optical switch 5-2 and itswavelength is multiplexed with the signal light ray 2-7 outputted fromthe transmitter 4-2 of external device 4-1 in a supervisory controllight controlling unit 2-2. The resulting signal light ray passesthrough the transmission path 2-1 and is then inputted to the adjoiningadjacent OADM device 1-1. The signal light ray 2-7 inputted to the OADMdevice 1-1 is split from the supervisory light ray 2-6 in thesupervisory control light controlling unit 2-2 of the adjacent OADMdevice 1-1, passed subsequently through the 2×2 optical switch 5-1 so asto be inputted to the wavelength multiplexer unit 2-5 and thereafter,inputted to the receiver 4-3 of external device 4-1. The wavelength ofsupervisory light ray 2-6 is split from the signal light ray 2-7 bymeans of the supervisory control light controlling unit 2-2 andthereafter, passed through the 2×2 optical switch 5-2 so as to beinputted to the control light processing unit 2-4-1.

Turning to FIG. 6, an OADM device configuration will be described inwhich OADM devices 1-1 located remotely from each other are combinedwith a supervisory control terminal 6-1 for remotely controlling theOADM devices 1-1. The supervisory control terminal 6-1 is connected to asupervisory control unit 2-4 of each OADM device and with a command fromthe supervisory control terminal 6-1, 2×2 optical switches 5-1 and 5-2mounted on the OADM device 1-1 installed remotely from the terminal 6-1can be driven. Namely, the 2×2 optical switch at an arbitrary locationcan be driven remotely by the supervisory control terminal 6-1 to ensurethat the optical loop back function to be described later can be carriedout at the arbitrary location. To add, the supervisory control terminal6-1 in FIG. 6 is connectable to the OADM device 1-1 locatable at anarbitrary location and therefore, by using the OADM device, the opticalloop back function to be detailed later can also be practiced at thearbitrary location. Further, the supervisory control unit 2-4 can alsobe provided self-controllably with the function to execute the opticalloop back to be described later.

The operation of the 2×2 optical switches 5-1 and 5-2 to be utilized inthe configuration of FIG. 5 will be described with reference to FIGS. 7Aand 7B. Each of the 2×2 optical switches 5-1 and 5-2 has four ports A,B, C and D and all of the ports can be used for input and output. Theoptical switch is conditioned internally to have at least two statuses,in one of which the port A and the port B are connected together and theport C and the port D are connected together and in the other of whichthe port A and the port D are connected together and the port C and theport B are connected together. By using the 2×2 optical switches 5-1 and5-2 operating as above, the optical loop back function is materialized.In normal operation status, the optical switch is switched as shown inFIG. 7A to pass a signal from a port (for example, A) opposing one endof the signal path of OADM device in the signal light ray transmissiondirection to a port (for example, B) opposing the other end. When theloop back is necessary, the optical switch is switched as shown in FIG.7B to fold a signal from a port (for example, A) opposing one end of thesignal path of OADM device in the signal light ray transmissiondirection to a port (for example, D) for transmission of the signal tothe one end. By folding the signal inputted to the optical switch asshown in FIG. 7B, the optical loop back function can be materialized.

Referring now to FIGS. 8A and 8B, the operation of the OADM device 1-1having the optical loop back function set up by using the operation ofthe 2×2 optical switch explained in connection with FIGS. 7A and 7B willbe described. Illustrated in FIG. 8A is the normal status of 2×2 opticalswitches 5-1 and 5-2 in which a signal light ray 2-7 inputted from theright passes through the optical switch 5-1 from port A to port B,reaching the wavelength multiplexer unit 2-5. On the other hand, asignal light ray 2-7 outputted from the wavelength multiplexer unit 2-5passes through the 2×2 optical switch 5-1 from port C to port D andoutputted to the right. Further, the wavelength of a supervisory lightray 2-6 is split by means of the supervisory control light controllingunit 2-2 and passes through the 2×2 optical switch 5-2 from port A toport B so as to be inputted to the control light processing unit 2-4-1.A supervisory light ray 2-6 outputted from the control light processingunit 2-4-1, on the other hand, passes through the 2×2 optical switch 5-2from port C to port D, undergoes multiple-wavelength in the supervisorycontrol light controlling unit 2-2 and is then outputted to the right.Illustrated in FIG. 8B are the statuses of 2×2 optical switches 5-1 and5-2 under the condition that the loop back function is operated, showingthat a signal light ray 2-7 inputted from the right undergoes loop backfrom port A to port D in the optical switch 5-1 and it is delivered asit is to an originator of the signal light ray 2-7. A signal light ray2-7 outputted from the wavelength multiplexer unit 2-5 undergoes loopback in the 2×2 optical switch 5-1 from port C to port B and isdelivered to the input originator of the signal light ray 2-7. Further,a supervisory light ray 2-6 inputted from the right undergoes wavelengthsplitting in the supervisory control light controlling unit 2-2,undergoes loop back in the 2×2 optical switch 5-2 from port A to port D,again undergoes multiple-wavelength in the supervisory control lightcontrolling unit 2-2 and thereafter it is delivered to an inputoriginator. Further, a supervisory light ray 2-6 outputted from thecontrol light controlling unit 2-4-1 undergoes loop back in the 2×2optical switch 5-2 from port C to port B and it is again inputted as itsis to the control light controlling unit 2-4-1.

By using the 2×2 optical switches explained in connection with FIGS. 7Aand 7B in this manner, the loop back function can be materialized on theoptical level as explained in connection with FIGS. 8A and 8B.

Next, the procedures for actually identifying an interval in which afault develops in the OADM device with the loop back function will bedescribed.

Illustrated in FIG. 9 is an instance where a fault 9-1 occurs betweentwo adjacent OADM devices. Since a signal light ray 2-7 to be inputtedto the receiver 4-3 of external device 4-1 and a supervisory light ray2-6 to be inputted to the control light processing unit 2-4-1 cannot betransmitted, detection is executed to determine which point the faultdevelops at. But, under the condition that the present loop backfunction is not applied, it cannot be identified which location on thecommunication path the fault occurs at, with the result that an actionto recover the path from the fault generation cannot be taken. Contraryto this, when the loop back is to be executed through the 2×2 opticalswitches 5-1 and 5-2, a signal light ray 2-7 and a supervisory light ray2-6 pass through such a route as will be explained with reference toFIG. 10. In the event that the signal light ray 2-7 to be inputted tothe receiver 4-3 of external device 4-1 and the supervisory light ray2-6 to be inputted to the control light processing unit 2-4-1 cannot betransmitted, the fault occurrence is to be detected and therefore, theoptical switch is driven to switch over the loop back ON/normaloperation of the loop back function. More specifically, in order toidentify a fault developing location, the optical switch is switchedover with a command from the supervisory control terminal or by means ofthe supervisory control unit and the presence/absence of a foldingsignal is examined.

When loop back setting is carried out in the 2×2 optical switch 5-1 inthe left side OADM device 1-1, normal inputting of the signal light ray2-7 to the receiver 4-3 of external device 4-1 can be confirmed andtherefore, it can be known that a fault does not occur between theexternal device 4-1 and the wavelength multiplexer unit 2-5. Similarly,when loop back setting is carried out in the 2×2 optical switch 5-1 inthe right side OADM device 1-1, normal inputting of the signal light ray2-7 to the receiver 4-3 of external device 4-1 can be confirmed andtherefore, it can be known that a fault does not occur either betweenthe external device 4-1 and the wavelength multiplexer unit 2-5 in theright side OADM device 1-1 as in the case of the left side OADM device1-1. Further, in the left side and right side OADM devices 1-1, asupervisory light ray 2-6 can also be received no ally in the controllight controlling unit 2-4-1 by setting the loop back to the 2×2 opticalswitch 5-2 and therefore, it can be found that a fault does not occur inthe route for the supervisory light ray 2-6. Setting of the loop backfunction for fault detection in the OADM device can be rendered ON ineither the optical switch 5-1 for signal light ray or the optical switch5-2 for supervisory light ray or in both of them. With the both opticalswitches are involved for setting, a decision can be made as to whichone of the signal light ray route and the supervisory light ray routethe fault occurs in.

Illustrated in FIG. 11 is an instance for explaining routes of signalgat ray 2-7 and supervisory light ray 2-6 when the loop back is set toonly the 2×2 optical switch 5-1 in the right side OADM device 1-1, withthe other 2×2 optical switch 5-2 and the 2×2 optical switches 5-1 and5-2 in the left side OADM device 1-1 being set to normal operation. Thesupervisory light ray 2-6 outputted from the control light processingunit 2-4-1 on the right side does not reach the control light processingunit 2-4-1 of the left side OADM device under the influence of a faultlocation 9-1. Then, the signal light ray 2-7 outputted from thetransmitter 4-2 in the left side external device 4-1 reaches the rightside OADM device 1-1 and undergoes the loop back by means of the 2×2optical switch 5-1 but does not reach the receiver 4-3 in the left sideexternal device 4-1. Under the conditions, it can be identified that afault occurs in a transmission path from the right side OADM device 1-1to the left side OADM device 1-1. Thus, with the two OADM devicesconfigured, a decision can be made as to whether a fault occurs in thetransmission path between the two OADM devices by confirming whether ornot a signal transmitted to the opposing OADM device in which the loopback function of at least part of the optical switches is set to ON isfolded and transmitted. Here, if the external device 4-1 is notconnected to the OADM device 1-1 and so the OADM device does not havesolely the function to transmit the signal light ray 2-7, the faultdetection based on the confirmation of the presence/absence of foldingof the signal light ray 2-7 will sometimes fail during fault separation.In the event that a fault developing location 9-1 explained inconnection with FIG. 11 exits when no external device 4-1 is connectedto the OADM device 1-1, for example, the signal light ray 2-7 does notat all exist in essential in the FIG. 11 configuration and the faultdeveloping location 9-1 cannot be identified.

Referring now to FIG. 12, a second embodiment will be described which isconfigured when a signal light ray is not inputted from an externaldevice 4-1. In FIG. 12, a loop back test unit 12-10 carrying a lightsource 12-6 of a test light ray used for execution of a loop back testis connected to an OADM device 1-1. Structurally, two OADM devices 1-1opposing to each other through the medium of two transmission paths 2-1are arranged. The loop back test unit 12-10 includes an optical splitter12-1, an optical switch 12-2, tunable filters 12-3, a 2×2 optical switch12-4, optical splitters 12-5, the light source 12-6 and input/outputsections 12-9. The light source 12-6 is constituted by a transmitter12-7 and a receiver 12-8. The optical switch 12-4 can be driven remotelywith a command from the aforementioned supervisory control terminal 6-1connected to the supervisory control unit 2-4 or the supervisory controlunit 2-4 can drive self-controllably the optical switch 12-4.

Turning to FIG. 13, an example of loop back test using the secondembodiment explained in connection with FIG. 12 is illustrated. When the2×2 optical switches 5-1 and 12-4 are all set to normal operation, atest light ray 13-1 transmitted from the transmitter 12-7 of lightsource 12-6 in the right side loop back test unit 12-10 is inputted tothe receiver 12-8 of light source 12-6 in the left side loop back testunit 12-10. On the other hand, when the 2×2 optical switch 12-4 is setto loop back function ON, a test light ray 13-5 transmitted from thetransmitter 12-7 of light source 12-6 in the right side loop back testunit 12-10 undergoes loop back in the 2×2 optical switch 12-4 mountedinside the right side loop back test unit 12-10 and folded inside itsown unit and it is so returned as to be inputted to the receiver 12-8 oflight source 12-6 to make confirmation as to whether the test light ray13-5 is received normally. Next, setting of the 2×2 optical switch 12-4of loop back test unit 12-10 is returned to normal operation and the 2×2optical switch 5-1 mounted in the OADM device 1-1 is set to loop backfunction ON. In this case, a test light ray 13-4 outputted from thelight source 12-6 is looped back in the 2×2 optical switch 5-1 andinputted to the receiver 12-8 of light source 12-6, making aconfirmation as to whether the test light ray 13-4 is received normally.Further, setting of the 2×2 optical switch 5-1 mounted in the OADMdevice 1-1 is returned to normal operation and the 2×2 optical switch5-1 mounted in the opposing OADM device 1-1 is set to loop back functionON. In this case, a test light ray 13-3 outputted from the light source12-6 is looped back by means of the 2×2 optical switch 5-1 mounted inthe opposing OADM device 1-1 and inputted to the receiver 12-8 of lightsource 12-6, making confirmation as to whether the test light ray 13-3is received normally. Next, the 2×2 optical switch 5-1 mounted in theopposing OADM device 1-1 is set to loop back OFF, that is, normaloperation and the 2×2 optical switch 12-4 mounted in the opposing loopback test unit 12-10 is set to loop back ON. In this case, a test signalray 13-2 outputted from the light source 12-6 is looped back by means ofthe 2×2 optical switch mounted in the opposing loop back test unit 12-10and inputted to the receiver 12-8 of light source 12-6, makingconfirmation as to whether the test light ray 13-2 is received normally.

The above loop back test is executed by using only the light source 12-6mounted in the right side loop back test unit 12-10 but by applying asimilar test to the opposing loop back test unit 12-10, a faultdeveloping location can be identified in the OADM device not connectedwith the external device 4-1. It is to be noted that the variablewavelength filter 12-3 arranged on the transmission side needs tocoincide with the wavelength of test light ray outputted from thetransmitter 12-7 of light source 12-6 and the variable wavelength filter12-3 arranged on the reception side needs to coincide with thewavelength of test light ray expected to be inputted to the receiver12-8 of light source 12-6. When the present tunable filter 12-3 isarranged on the transmitter side, a test light ray having an unintendedwavelength is prevented from being inputted owing to a fault oftransmitter 12-7 of test light ray 13-5. Further, when the presenttunable filter 12-3 is arranged on the reception side, even with anunexpected wavelength inputted to the receiver 12-8 for test light ray13-5 externally thereof, the receiver 12-8 can be prevented fromoperating erroneously and detecting erroneously.

Turning to FIG. 14, an example of configuration of a third embodiment inwhich a single loop back test unit 12-10 is connected to a plurality ofOADM devices 1-1 will be described. The loop back test unit 12-10 is thesame as that explained in connection with FIG. 12 but the single loopback test unit 12-10 is connected to the plural OADM devices 1-1.Consequently, the loop back test unit 12-10 is shared by the plural OADMdevices and the configuration is advantageous from a standpoint of costreduction. In FIG. 14, the single loop back test unit 12-10 is connectedto three OADM devices 1-1 but it can also be connected to more thanthree OADM devices 1-1.

The operation of the optical splitter 12-1 in the third embodiment willbe described with reference to FIG. 15. The optical splitter 12-1mounted in the loop back test unit 12-10 is arranged on the transmissionside of light source 12-6. Therefore, a test light ray outputted fromthe transmitter 12-7 is split in the optical splitter 12-1, so that atest light ray 15-1 directed to the upper OADM device 1-1, a test lightray 15-2 directed to the middle OADM device 1-1 and a test light ray15-3 directed to the lower OADM device 1-1 are inputted simultaneously.This ensures that the loop back test can be carried out at a time inrespect of the plural OADM devices. The optical splitter 12-1 can bemerged into the optical switch 12-2 but in this case, a test light rayis selectively inputted to only one, subject to execution of loop backtest, of the plural OADM devices connected to the loop back test unit12-10. Namely, when the test is to be executed for the upper OADMdevice, the optical switch unit 12-2 is switched to the upper OADMdevice so as to send the test light ray 15-1 to the upper OADM devicebut no test light ray is sent to the middle and lower OADM devices 1-1.

The operation of the optical switch 12-2 in the third embodiment will bedescribed with reference to FIG. 16. The optical switch 12-2 mountedinside the loop back test unit 12-10 is arranged on the reception sideof the light source 12-6. Therefore, the optical switch 12-2 selects anyone of test light rays 16-1, 16-2 and 16-3 from the upper, middle andlower OADM devices 1-1, respectively and inputs a test light ray as aresult of the selection to the receiver 12-8. Thus, a problem can beprevented in which when loop back tests are carried out from the pluralOADM devices at a tine plural test light rays 16-1, 16-2 and 16-3 areinputted at a time to the light source 12-6, thus failing to make adecision as to which OADM the test light ray is inputted from.

As set forth so far, the result of loop back function is decided(detected) in accordance with the presence/absence of the foldingsignal. In other words, the presence/absence of loop back is decided byreceiving signals folded and returned by the optical switch at thereceiver 4-3 of external device 4-1, the control light processing unit2-4-1 and the receiver 12-8 of loop back light source 12-6.

It should be further understood b those skilled in the art that althoughthe foregoing description has been made on embodiments of the invention,the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. An optical transmission apparatus comprising: a wavelengthmultiplexer unit for multiplexing wavelengths of input/output lightrays; a first optical switch for inputting/outputting light rays to/fromsaid wavelength multiplexer unit; a control light processing unit forinputting/outputting control light rays; a control light controllingunit for performing multiplexing or splitting between the control lightray and the light ray delivered out of said wavelength multiplexer unit;and a supervisory control unit for controlling on/off of the function tofold the inputted light ray in respect of said first optical switch. 2.An optical transmission apparatus according to claim 1 furthercomprising a second optical switch arranged between said control lightprocessing unit and said control light controlling unit, wherein saidsupervisory control unit further controls on/off of the function to foldthe inputted light ray in respect of said second optical switch.
 3. Anoptical transmission apparatus according to claim 1, wherein saidoptical transmission apparatus is connected to another opticaltransmission apparatus through transmission paths to mutuallytransmit/receive light rays to/from the different optical transmissionapparatus.
 4. An optical transmission apparatus according to claim 3,wherein said wavelength multiplexer unit multiplexes input/output signallight rays and said first optical switch is a 2×2 optical switch adaptedto switch signal light rays transmitted to the different opticaltransmission apparatus and signal light rays received from the differentoptical transmission apparatus as well.
 5. An optical transmissionapparatus according to claim 3 further comprising a second opticalswitch arranged between said control unit and said control lightcontrolling unit, wherein said supervisory control unit further controlson/off of the function to fold the inputted light ray in respect of saidsecond optical switch, and said second optical switch is a 2×2 opticalswitch adapted to switch control light rays transmitted to the differentoptical transmission apparatus and control light rays received from saiddifferent optical transmission apparatus as well.
 6. An opticaltransmission apparatus according to claim 1 further comprising a lightsource of a test light ray and a third optical switch forinputting/outputting light rays to/from said light source, whereinon/off of the function to fold inputted light rays is controlled by saidsupervisory control unit in respect of said third optical switch.
 7. Anoptical transmission apparatus according to claim 2 further comprising alight source of a test light ray and a third optical switch forinputting/outputting light rays to/from said light source, whereinon/off of the function to fold inputted light rays is controlled by saidsupervisory control unit in respect of said third optical switch.
 8. Anoptical transmission apparatus according to claim further comprising alight source of a test light ray and a third optical switch forinputting/outputting light rays to/from said light source, wherein saidthird optical switch is a 2×2 optical switch whose function to foldinputted light rays is on/off controlled by said supervisory controlunit and which is adapted to switch both of a test light ray transmittedto said different optical transmission apparatus and a test light rayreceived from said different optical transmission apparatus.
 9. Anoptical transmission apparatus according to claim 6 further comprising asplitter arranged between said third optical switch and said wavelengthmultiplexer unit and adapted to split said test light ray.
 10. Anoptical transmission apparatus according to claim 6 further comprising afourth optical switch arranged between said third optical switch andsaid wavelength multiplexer unit and adapted to switch said test lightray outputted from said wavelength multiplexer unit.
 11. An opticaltransmission apparatus according to claim 1, wherein said supervisorycontrol unit receives from a supervisory control terminal arrangedexternally a command for setting said first optical switch.
 12. Anoptical transmission apparatus according to claim 6 further comprising atunable filter arranged between said third optical switch and saidwavelength multiplexer unit.
 13. An optical transmission method based onan OADM device having a wavelength multiplexer unit, a first opticalswitch, a control light processing unit and a control light controllingunit, comprising the steps of: multiplexing wavelengths of input/outputlight rays by means of said wavelength multiplexer unit; switching lightrays outputted from said wavelength multiplexer unit by means of saidfirst optical switch; multiplexing light rays switched by said firstoptical switch with a control light ray outputted from said controllight processing unit by means of said control light controlling unitand transmitting a resulting light ray to an opposing OADM device;splitting light rays received from said opposing OADM device by means ofsaid control light controlling unit into a control light ray and anotherlight ray; and inputting the different light ray to said first opticalswitch and switching the inputted light ray, wherein said first opticalswitch folds the inputted light ray and returns it to an inputoriginator when the function to fold the inputted light ray is ON. 14.An optical transmission method according to claim 13, wherein said OADMdevice is connected to an external device and transmits/receives asignal light ray to/from said external device, said method furthercomprising the step of making a decision as to whether generation of afault is present or absent in a transmission path in respect of a signallight ray transmitted from said external device by deciding whether ornot the signal light ray transmitted from said external device is foldedby the folding function of said first optical switch.
 15. An opticaltransmission method according to claim 13 further comprising the step ofperforming switching between a control light ray outputted from saidcontrol light processing unit and said different light ray split bymeans of said control light controlling unit by using a second opticalswitch arranged between said control light processing unit and saidcontrol light controlling unit, wherein when the function to fold aninputted light is ON, said second optical switch folds the inputtedlight ray and transmits it to an input originator.
 16. An opticaltransmission method according to claim 15, wherein said OADM device isconnected to an external device so as to transmit/receive a signal lightray to/from said external device, said method further comprising thestep of making a decision as to whether generation of a fault is presentor absent in a transmission path in respect of a control light raytransmitted from said control light processing unit by deciding whetherthe control light ray transmitted from said control light processingunit is folded by the folding function of said second optical switch.17. An optical transmission method according to claim 13 furthercomprising the steps of: outputting a test light ray from a lightsource; and switching the test light ray by a third optical switchadapted to input/output the test light ray, wherein when the function tofold the inputted light ray is ON, said third optical switch folds theinputted light ray and transmits it to an input originator.
 18. Anoptical transmission method according to claim 17 further comprising thestep of making a decision as to whether generation of a fault is presentor absent in a transmission path in respect of a test light rayoutputted from said light source by deciding whether the test light raytransmitted from said light source is folded by the folding function ofsaid first optical switch.